Environmental variables regulating soil carbon dioxide efflux following clear-cutting of a Pinus radiata D. Don plantation

The regulation of soil surface CO2 efflux (F-s) from a dryland Pinus radiata clear-cut in New Zealand (658-mm annual rainfall, 50-mm soil water storage over 0.5-m depth of soil) was quantified using a multiplicative, soil moisture and temperature constraint model. Model parameters were determined from measurements of F-s by eddy covariance and by two portable CO2 analyzers equipped with soil chambers during 8 consecutive days in November 1995. F-s from the two chamber systems agreed consistently, but agreement between chamber and eddy covariance measurements was confined to a very small flux-range (1.5-2.5 mu mol m(-2) s(-1)). When the soil surface was wet and wind speed (u, a surrogate for static pressure fluctuations) was high, eddy covariance fluxes were >3 mu mol m(-2) s(-1) (maximum 5.0 mu mol m(-2) s(-1)), significantly higher than the chamber values. Including u as a third constraint in the model facilitated good predictions of F-s over the entire range of environmental conditions. Daily F-s during the study period ranged from 0.8 to 2.9 g C m(-2). Driven by weather data from the forest fire station and subroutines for soil water balance and temperature, modeled F-s for the year ending in June 1996 was 0.37 kg C m(-2). This year had below average (547 mm) rainfall. For a year with 640-mm rainfall, a significant increase of annual F-s (0.44 kg C m(-2)) demonstrated the severe restriction of soil microbial activity by moisture deficit at dryland sites. Moreover, when the soil was adequately watered, F-s was frequently restricted by low wind speed.